ADP-ribosylating bacterial exotoxins are widely known. Examples include diphtheria toxin (Corynebacterium diphtheriae), exotoxin A (Pseudomonas aeruginosa), cholera toxin (CT; Vibrio cholerae), heat-labile enterotoxin (LT; E. coli) and pertussis toxin (PT).
The toxins catalyse the transfer of an ADP-ribose unit from NAD+ to a target protein. CT, for instance, transfers ADP-ribose to a specific arginine side chain of the α subunit of GS, which blocks the ability of GS to hydrolyse GTP to GDP. This locks the protein in its ‘active’ form, so adenylate cyclase activity is permanently activated. Cellullar cAMP levels rise, leading to the active transport of ions from the cell and the loss of water into the gut [1].
The toxins are typically divided into two functionally distinct domains—A and B. The A subunit is responsible for the toxic enzymatic activity, whereas the B subunit is responsible for cellular binding. The subunits might be domains on the same polypeptide chain, or might be separate polypeptide chains. The subunits may themselves be oligomers e.g. the A subunit of CT consists of A1 and A2 which are linked by a disulphide bond, and its B subunit is a homopentamer. Typically, initial contact with a target cell is mediated by the B subunit and then subunit A alone enters the cell.
Crystal structures [2] are known for LT [3], CT [4] and PT [5].
The toxins are typically immunogenic, and have been proposed for use in acellular vaccines. One problem, however, is that the proteins retain their toxic activity in the vaccines. To avoid this problem, site-directed mutagenesis of key active site residues has been used to remove toxic enzymatic activity whilst retaining immunogenicity [e.g. refs. 6 (CT and LT), 7 (PT), 8 etc.]. Current acellular whooping cough vaccines include a form of pertussis toxin with two amino acid substitutions (Arg9→Lys and Glu129→Gly; ‘PT-9K/129G’ [9]).
As well as their immunogenic properties, the toxins have been used as adjuvants. Parenteral adjuvanticity was first observed in 1972 [10] and mucosal adjuvanticity in 1984 [11]. It was surprisingly found in 1993 that the detoxified forms of the toxins retain adjuvanticity [12].
It is an object of the invention to provide further ADP-ribosylating bacterial toxins.